Method and magnetic resonance apparatus for quantitative, highly reproducible tissue differentiation

ABSTRACT

The present invention relates to parameter quantification for reproducible characterization of measured magnetic resonance signal variations in biological tissues due to variation in contrast-weighting levels. The method uses comprehensive sampling and higher-order model analysis to attain a more complete description of the signal variation at high signal-to-noise ratio. The signal variation described by the higher-order model fit is subsequently used for retrospective fit analysis based on a more basic model and a flexible sampling pattern. This approach greatly facilitates reproducibility of parameter quantification, since sampling inconsistencies can readily be accounted for.

BACKGROUND OF THE INVENTION

The invention generally relates to methods for computing pathologicallysignificant parameter maps associated with signals measured in magneticresonance imaging MRI experiments. With MRI, images can benon-invasively obtained from the interior of an object or a human bodylocated inside a magnetic resonance (MR) scanner. These images arecharacterized by outstanding spatial resolution and tissue contrast. Theresulting tissue contrast is determined by scan parameter settings thatchange the sensitivity to the recovery or decay of the observed tissuesignal. More particularly, an exponential signal recovery or decay canbe related to magnetic field-dependent T1 relaxation, T2 relaxation orT2* relaxation. In biological tissues, the field-dependent relaxationoccurs with time constants ranging from fractions of a millisecond toseveral seconds. With suitable encoding, Brownian motion, i.e., thephysical process of molecular diffusion, can also generate signals thatdecay exponentially with encoding strength. Typically, for any of thesephysical processes, unless simple objects are studied, like liquids, theobserved signal variation is multi-exponential by nature. For example,in biological tissues, even at high spatial resolution, the compositesignal of multiple compartments at a cellular level, each with acharacteristic exponential signal variation, is observed.

A great research effort has been made to design acquisition protocolsand analysis approaches that permit a decomposition of thismulti-exponential signal variation into distinct exponential components.Meanwhile, for typical clinical applications, such comprehensivemeasurement and detailed analysis is not practical. Nevertheless, thereis a great interest to obtain simple-to-interpret quantitative mapsbased on a mono-exponential analysis of the complex signal behavior. Innumerous studies, involving different organs and pathologies, it hasbeen shown that such maps can be helpful to distinguish normal fromabnormal tissue and even to categorize levels of abnormality andassociated risk for disease progression. Such maps may also be helpfulin artificial-intelligence supported classification. However, themono-exponential signal analysis used to derive these maps cannotadequately describe the observed more complex signal variation ofbiological tissues. A consequence of this inadequate modeling of thesignal variation is that the quantitative maps exhibit a pronounceddependence on acquisition parameters and, therefore, lackreproducibility in a clinical environment that uses a wide variety ofacquisition protocols and MR equipment.

SUMMARY OF THE PRIOR ART

Tissue differentiation, localization and characterization always havebeen basic goals of MRI. Indeed, the potential to distinguish betweennormal tissue and tumor tissue using MRI techniques was recognizedseveral decades ago. Early on it was realized that the spin-lattice, socalled “T1”, and especially the spin-spin, so called “T2”, relaxationparameters are different between normal and cancerous tissue.Accordingly, by appropriately mapping the various T1 and/or T2relaxation times determined from the MR signal of various voxels in ananatomical slice of interest as relative image amplitudes, it waspossible to create images generally showing the demarcation of tumortissue from adjacent normal tissue. More recently, mapping of the faster“T2*” relaxation parameter with so called gradient echo sequences hasbeen proposed to measure tissue iron content and levels of deoxygenatedhemoglobin. The observed signal recovery related to T1 relaxation, andsignal decay related to T2 or T2* relaxation can be described withmulti-exponential functions. Yet another important method that relies onthe quantification of multi-exponential signal decays is MRdiffusion-weighted imaging, which can be used to visualize the Brownianmotion of molecules, e.g., water molecules, in tissues. In this regard,the diffusion-weighted images, but also the, so-called “apparentdiffusion coefficient” (ADC) maps, as a quantitative measure of thediffusion-related signal decay, can provide useful information about thepresence of acute stroke, fluid-filled cysts or tumor lesions.

In the intervening time period, methods of obtaining T1, T2, T2* ordiffusion-weighted images and related quantitative maps using MRItechniques have vastly improved. In addition, a large amount ofexperience has been gained in the in vivo application of these methodsand the added use of contrast agents has resulted in even betterdiagnostic possibilities. In fact, methodology has evolved to the pointthat the demarcation of tissue boundaries is considered to be basicallyconventional. Nevertheless, the categorization of tissue as normal orabnormal based on quantitative parameter maps is not entirelysuccessful, since it was observed that the quantitative thresholdsadopted for such categorization were not reproducible among thedifferent imaging protocols and MR systems.

The most widespread clinical and research use of quantitative signalanalysis in MR occurs in the context of diffusion imaging. It is alsothe area, where understanding the observed signal decays requires themost complex models and where a directional dependence plays a role. Butmany aspects of the problems that can arise in the analysis of suchsignal decays, are equally applicable to T1, T2, or T2* relaxationmapping. To better understand the analysis concepts involved, it will beinstructive to first generally discuss some basics, such as theacquisition of diffusion-weighted signals with MRI and how the measuredsignal decays relate to the tissue under observation. First, theconcepts of isotropic diffusion, the so-called “diffusion coefficient”,and the measurement of the “diffusion coefficient” with MR will bepresented in a generalized manner. Second, the concepts of the extensionof the definition of diffusion to so-called “anisotropic” diffusion, andthe characterization of diffusion with a diffusion tensor, rather than asingle coefficient, will be introduced. Third, the effects of havingmultiple compartments with different diffusion properties within adetected voxel signal, such as the combined effect of blood perfusion inthe micro-circulatory system and water diffusion in the surroundingtissue will be presented. Finally, the phenomenon of a departure fromthe normally adopted MR signal behavior when the diffusion encodingrange is extended, will lead to a discussion of the present invention.

First, with regard to isotropic diffusion and its measurement using MR,it will be recognized that in a pure liquid such as water at roomtemperature, the individual water molecules are in constant motion dueto the phenomenon of thermal agitation. This phenomenon is commonlyreferred to as “Brownian motion”. The so-called “diffusion coefficient”(herein sometimes referred as “D”) is a measure of this molecularmotion, and it can be determined with MR techniques.

More particularly, a magnetic field gradient can be used to “tag” atomiclevel spins in a sample according to their location in space at the timeof the application of a first magnetic field gradient to the sample. Asecond gradient, applied at a later time, then serves to probe how far,on average, the individual spins have moved between the time of thefirst gradient application and the time of the second gradientapplication. In the ideal case, these magnetic field gradients areapplied in brief, strong bursts separated by a common well-defined timeperiod. In clinical MR systems, the gradients are applied for a moderateduration of several milliseconds to several tens of milliseconds, andthe leading edges of the respective bursts are separated by delays ofsimilar, but slightly longer duration. The timing parameters δ and Δrefer to the duration of the diffusion gradient pulses and to theseparation of their leading edges, respectively. Under these conditions,the diffusion encoding level, i.e., the so-called “b-factor”, is definedby the following relationship:b=γ ² G ²δ²(Δ−δ/3)where γ is the gyromagnetic ratio (42.58 MHz/Tesla for protons) and G isthe magnetic field gradient amplitude. The diffusion-encoding gradientwaveforms are usually optimized for short duration within theconstraints of desired diffusion encoding and apparatus-specificgradient amplitude and slew rate limitations. The short duration ensuresshortest possible echo time with maximum attainable signal-to-noiseratio (SNR).

In an experiment with one gradient pulse placed prior to and the otherfollowing the 180° radio frequency (RF) inversion pulse of a spin echosequence (90° RF slice select—TE/2—180° RF inversion—TE/2—signalacquisition), the signal S in dependence of diffusion encoding b thatresults from the spin-echo measured at echo time TE in a simpleisotropic diffusion environment like a liquid follows a monoexponentialrelationship:S(b)=S ₀ exp(−bD).In this relationship, S₀ depends upon machine constants, the spin-spinrelaxation time T₂, the spin-lattice relaxation time T₁ in anyexperiment that repeats measurements every repetition time period TR,and the spin density ρ. Specifically, the diffusion coefficient D may bemeasured by making multiple measurements of S as a function of b,plotting the natural logarithm of S vs. b and then performing a linearregression whose slope provides the experimental measurement of D.Alternatively and preferably, a non-linear least-square regressionanalysis can be used to directly infer the fitting exponential functionwithout performing the logarithm operation. The value of b is mostconveniently varied by keeping the gradient timings fixed andincrementing the amplitude G of the magnetic field gradient.

As will be seen from FIG. 1 , the logarithmic decay of signal intensityfrom neat solutions of water as a function of b measured on a clinicalscanner follows a straight line. This is indicative of a monoexponentialsignal decay above the base line noise level. In this example, theactual diffusion coefficient measured from the slope of the decay shownabove the base line noise values is in excellent agreement withpublished literature values. Hence, for isotropic diffusion in neatliquids, it may be said that the logarithm of the intensity of the MRsignal varies linearly with b above a given noise threshold.

Second, the extension of the foregoing concepts to the measurement oftissue water diffusion within the context of MRI led to certainadjustments in the above-stated theory. It was quickly realized that incertain organs like the brain, preferred directions of water diffusionexist. More particularly, diffusion along one direction, as selected bythe direction of the magnetic field gradient vector could be differentthan the diffusion along another direction. In the brain, this lack ofisotropy of the diffusion coefficient (the so-called “diffusionanisotropy”) is attributed to the presence of nerve fiber tracts alongwhich water is more free to move than it is in directions perpendicularto these tracts.

Indeed, in the light of the phenomenon of restricted or anisotropicdiffusion, it generally is agreed in the art that at least threeorthogonal directions of the diffusion sensitization gradient (which areindependent of the preferred directional diffusion) should be sampled togenerate trace images, i.e., maps of a rotationally invariant measure ofdiffusion. Further, a minimum of six directions must be sampled for eachvoxel, to determine the preferred direction of diffusion with thediffusion tensor formalism. More advanced formalisms have been developedfor use with acquisition protocols that sample much more than sixdirections. This approach can be useful to detect the presence andorientation of crossing fibers.

Thus, the current trend in the clinical implementation of diffusionimaging is to sample images of multiple sections in an organ, each atleast at a low and a high b-factor, the latter being typically on theorder of about 1000 s/mm². This high b-factor sampling commonly isrepeated for three orthogonal diffusion sensitizing gradient directions,if only a rotationally invariant measure of diffusion is needed, or atleast six non-collinear and non-coplanar diffusion sensitizing gradientdirections, if the assessment of diffusion anisotropy and preferreddirection of diffusion is planned. Nevertheless, despite additionalcomplexity added by the diffusion tensor formalism, the logarithmic plotof signal versus b-factor along a selected diffusion sensitizingdirection is still seen to follow a substantially monoexponentialbest-fit relationship.

Still other experiments, however, have suggested that themonoexponential signal decay vs. b-factor relationship just mentionedmay not be necessarily accurate. Thus in vivo studies of water diffusionin brain, liver and prostate have suggested that the signal decayvariation with b-factor is a biexponential function over a limitedb-factor range under 500 s/mm². Although the departure from a simplemono-exponential decay was found to be small, it is generally consideredto be attributable to perfusing blood. More specifically, blood in themicro-circulation has a very high diffusion coefficient that is notattributable to the normal, thermal Brownian motion associated with theremainder of the tissue water (i.e., water within and between the cells,but not in the vasculature). Consequently, there is a general consensusthat there is indeed a small, very quickly diffusing componentcontributing to signal decay at low b-factors under 300 s/mm² in bloodperfused organs.

To minimize the effect of perfusion on the measured diffusioncoefficient, routine clinical MR diffusion imaging is conducted atb-factors of between about 100 and 1000 s/mm². Diffusion coefficientmaps are then generated on a pixel-by-pixel basis assuming that the“best-fit” relationship between the MR signal and the b-factor is amonoexponential function (substantially as set forth above with regardto isotropic diffusion).

As will be seen from FIG. 2 , with an extended b-factor measurementrange, the diffusion-attenuated signal in tissue at higher b-factors isdistinctly higher as one might predict by looking only at signalsobtained up to a b-factor of 1000 s/mm² and then extrapolating to higherb-factors with a mono-exponential function. It is generally agreed uponthat the observed departure from a mono-exponential decay over thisextended decay stems from the presence of different diffusionenvironments within an image voxel. One obvious cause can be a partialvolume effect from a voxel containing both tissue and liquid, e.g., avoxel where tissue borders to cystic fluid. The diffusion within thecyst would be similar as in a neat solution of water at bodytemperature, but higher than in the adjacent tissue. Such partial volumecontributions of liquid pools can be eliminated with a suitable sequenceof excitation pulses that exploit differences in T1 relaxation. Withsuch preparation, signal decays are comparable to voxels with uniformtissue content, but still exhibit clearly non-mono-exponential signaldecay. The consensus is that in tissues at least two compartments can bedifferentiated, i.e., an extra-cellular compartment with hindereddiffusion and an intra-cellular compartment restricted by the cellmembrane.

Accordingly, higher-order signal decay models have been proposed thataccount for the presence of more than one diffusion compartment. One ofthe simplest models that describe a non-monoexponential diffusion signaldecay and which also reflects the nature of two compartments is thebiexponential model:S(b)=S ₀(f·exp(−bD _(f))+(1−f)·exp(−bD _(s)))In this equation, D_(f) and D_(s) describe the diffusion coefficients oftwo compartments with fast and slow diffusion, respectively. Thefraction f is used to account for the relative signal contribution ofeach compartment.

While this model has been highly successful to describe the observedtissue water signal decay at higher b-factors in different tissues,e.g., the extra and intra-axonal diffusion orthogonal to nerve fibers,it may not be sufficient to fully describe the observed signal decay. Ithas been suggested that diffusion within the cells is alsocompartmentalized, e.g., by organelles, such as the nucleus,mitochondria and vesicles, which are known to be surrounded by membranesthat impede the water exchange with their surroundings. One obviousextension of the biexponential model would be to add one or even morecompartments, each characterized by a different diffusion coefficient.However, it was soon realized that the prevailing noise in typical datawould usually not permit the finding of stable numerical solutions formore than two compartments. An inverse Laplace transform of the measureddecay signal would be the mathematical method to systematicallydecompose the multi-exponential signal decay into different components,each with a specific mono-exponential decay. However, such approachwould not be practical below SNRs of at least 1000:1 (where SNR ratiorefers to the SNR of the image with the lowest diffusion weighting), alevel that is at least off by a magnitude from what is observed intypical clinical exams.

Nevertheless, in order to account for a multitude of compartments withdifferent diffusion coefficients, it was then suggested to employ fitsthat rely on a continuous distribution of coefficients. A gammadistribution of diffusion coefficients, characterized by the shapeparameter k and the scaling parameter θ results in the followingparticularly simple relationship for the signal decay without anyexponential function:

${S(b)} = \frac{S_{0}}{\left( {1 + {\theta b}} \right)^{k}}$This relationship describes observed signal decays in tissues remarkablywell. Another distribution of diffusion coefficients results when thesignal decay relationship is described with a stretched exponentialfunction:S(b)=S ₀ exp(−(bD)^(α))Where a stretching coefficient α introduces the deviation from a simplemono-exponential decay. The resulting distribution is characterized by arelatively large contribution of fast diffusion compartments, whichmakes it suitable to account for the perfusion effect discussed earlier.

Yet, another widely used model is based on the analysis of changes inthe spin displacement probability distribution. In the case of amono-exponential decay, the spin displacement probability distributionis a normal distribution. The deviation from a normal distribution isexpressed by the excess kurtosis factor K and the relationship betweenmeasured signal S and b can be approximated with a Taylor expansion ofthe decaying exponential. It is usually truncated to a second-orderpolynomial in b according to the following relationship:

${S(b)} = {S_{0}{\exp\left( {{{- b} \cdot {ADC}_{K}} + {b^{2} \cdot {ADC}_{K}^{2} \cdot \frac{K}{6}}} \right)}}$

The variable ADC_(K) represents the apparent kurtosis diffusioncoefficient. Since the Taylor expansion is truncated, the positivesecond order term starts to dominate at higher b-factors and, therefore,from a certain b-factor on an increase of the fitted signal, rather thana decay, can be observed. To ensure that S(b) is a monotonicallydecreasing function of the b-factor, an upper bound b≤3/(K·ADC_(K)) canbe derived.

For SNRs prevalent in clinical exams none of the models follows theobserved signal decay significantly and distinguishably better;especially if the b-factor range extension is moderate, e.g., up to 2000s/mm². As will be seen from FIG. 2 , for a wider range and SNRs inexcess of 200:1, however, the gamma distribution related model and thebiexponential model yield superior fits.

While at prevailing SNRs all of these higher-order model functionsdescribe the observed decays sufficiently well, there has not been awide-spread clinical adoption of diffusion measurements that wouldpermit such analyses. One major reason is that non-monoexponentialanalysis, in comparison to the basic monoexponential analysis, ischaracterized by diminished robustness in parameter estimation andsignificantly noisier parameter maps. Moreover, a multi-parametricassessment lacks the diagnostic simplicity of a single diffusioncoefficient map that results with monoexponential analysis.

Given that the diffusion signal decay in tissues shows a departure froman ideal monoexponential diffusion related signal decay at both low andhigh diffusion weighting, diffusion coefficients determined bymonoexponential signal analysis, are referred to as “apparent diffusioncoefficients” (ADC) rather than more generically as diffusioncoefficients D. As will be seen from FIG. 3 , if mono-exponentialanalysis of tissue water diffusion is performed for different maximumb-factors, it is quite obvious that ADC will depend on the choice of themaximum b-factor. It is important, however, to note that such variationof the ADC is also observed for measurements below 1000 s/mm², i.e., arange where, provided perfusion effects can be disregarded by choosing asuitable lowest b-factor, the mono-exponential function is considered anadequate fit. It can also be shown that the number of measurement pointsand the fitting method, i.e., linear-least square fitting afterlogarithmizing or direct non-linear least square fitting, influences theresult.

This clearly speaks against the universal establishment of ADC as ameasure to quantitatively categorize lesions, particularly since thereare no standardized b-factor measurement ranges. As can be seen fromFIG. 4 , even the establishment of consistent settings for b-factorswould not be successful, since b-factors will invariably vary within theimaging volume due to non-linearities of the magnetic field gradients,leading to a spatial ADC dependence.

SUMMARY OF THE INVENTION

The present invention uses retrospective monoexponential fitting tofunction values of non-monexponential (higher-order) fits applied to MRsignals measured at multiple weighting levels. Compared to the directmonoexponential fitting to measured signals that is conventionallyapplied, fundamental advantages arise. Firstly, the retrospectiveselection of weighting levels for which function values are determined,can be done with minimal restrictions. This implies the ability toreplicate any desired sampling protocol retrospectively, even if themeasured range of weighting levels is not consistent. Secondly, thenoise-related variance of a fit is typically much smaller than thenoise-related variance of the data the fit was applied to. This impliesthat both weighted images and parameter maps can be reconstructed thatexhibit much higher SNR than would result from using the measured dataor direct fits thereto. A prime application area is the field ofdiffusion-weighted Mill. Here, because the ADC determined with the newmethod exhibits minimal dependence on sampled range ofdiffusion-weighting levels, tissue ADC quantification errors atoff-center locations can be avoided by using actual diffusion-weightinglevels rather than nominal diffusion-weighting levels. Similarly, thislatitude in the range of measured diffusion-weighting levels can begainfully employed to harmonize diffusion imaging protocols in terms ofecho time and diffusion encoding time for systems with differentgradient hardware capability. Thus, the proposed method will permittruly reproducible ADC measurements, which can have great implicationsfor quantitative tissue characterization in Mill.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative graphical representation of the logarithmicdecay signal intensity from pure water at 17.5 C as a function ofb-factor;

FIG. 2 is an illustrative graphical representation of the logarithmicdecay signal intensity of diffusing water in a biological tissue(prostate tumor) as a function of b-factor, with mono-exponential andnon-monoexponential model fits;

FIG. 3 is an illustrative graphical representation of the logarithmicdecay signal intensity of diffusing water in a biological tissue (braintumor) as a function of b-factor, with a biexponential model fit thatfollows the measured signal decay and monoexponential fits that resultwith different maximum b-factors;

FIG. 4 is an illustrative graphical representation of the ADC measuredwith b-factors 0 and 1000 sec/mm² at different locations along themagnet principal axes x, y, and z in a water phantom surrounded withice-water and a known ADC value of 1.1 μm²/ms at 0 C temperature;

FIG. 5 is a high-level block diagram of an illustrative embodiment of aMill system suitable for use in the method of present invention;

FIG. 6 shows in one diagram over a common time axis the sequence ofmagnetic field pulses, radiofrequency pulses, and acquiredradiofrequency signals for image formation based on a prior art pulsesequence design for a single-shot 2D echo-planar readout usingdiffusion-weighting preparation on the system illustratively depicted inFIG. 5 . It may be mentioned that the illustrated time diagram is notdrawn to scale, i.e. the individual time sections and the amplitudes arenot drawn in accordance with their actual mutual size ratios;

FIGS. 7A-C are illustrative graphical representations of simulations atdiscrete b-factors and continuously evaluated analytic solutions of theaveraging effect in simulated normal prostate tissue (N) and simulatedtumorous prostate tissue (T) with different higher-order model fittingapproaches for a maximum diffusion weighting level b_(max)=1500 s/mm²(FIG. 7A), for a maximum diffusion weighting level b_(max)=2000 s/mm²(FIG. 7B), and for a maximum diffusion weighting level b_(max)=2500s/mm² (FIG. 7C);

FIGS. 8A-D are illustrative graphical representations of ADC maps of theprostate obtained in a patient with a conventional clinical protocol(FIG. 8A) and fitting with the proposed method using a kurtosis modelfunction and b_(max) 2000 s/mm² (FIG. 8B), using a biexponential modelfunction and b_(max) 2000 s/mm² (FIG. 8C), and using a gammadistribution model function and b_(max) 2000 s/mm² (FIG. 8D).

FIGS. 9A-D are illustrative graphical representations of ADC maps of theprostate obtained in a patient with a conventional clinical protocol(FIG. 9A) and fitting with the proposed method using a kurtosis modelfunction and b_(max) 1500 s/mm² (FIG. 9B), using a kurtosis modelfunction and b_(max) 2000 s/mm² (FIG. 9C), and using a kurtosis modelfunction and b_(max) 2500 s/mm² (FIG. 9D).

DETAILED DESCRIPTION OF THE INVENTION

The invention relies on the non-monoexponential analysis of diffusionsignal decays. This implies measuring a wider range of diffusionweighting levels than commonly applied in routine clinical diffusionimaging. With conventional ADC measurements, range and number ofdiffusion weighting levels, i.e., b-factors, are defined with theprotocol used during the acquisition. This choice cannot be modifiedafter the acquisition. With water diffusion measurements in biologicaltissues, unlike diffusion measurements in neat liquids, the presence ofmultiple diffusion compartments with distinct diffusion behaviorproduces multiexponential signal decays. These decays cannot beadequately described with the monoexponential model that is used in thecalculation of the ADC. A consequence of this deficient model is atremendous dependence of the resulting ADC on the range, distributionand number of diffusion weighting levels. Since there is no agreement onprotocols, tissue ADC values and recommended thresholds fordifferentiating tissues vary widely among sites. And even if there wassuch agreement, it would not hold for the measurement at off-centerpositions, since gradient non-linearities would lead to a differentdiffusion encoding than nominally given by the protocol. While it isobvious that prior collection of many diffusion weighting levels doespermit a selection of suitable encoding levels after the fact, thiswould constitute an inferior approach, since not necessarilymeasurements from all encoding levels would contribute to the finalresult, i.e., the ADC map. This approach is considered even more flawedin view of the fact that often signal averaging is employed to reach anacceptable SNR. Averaging is not only needed in order to attain ADC mapsof diagnostic quality, but also to generate diagnostically valuablediffusion-weighted images with adequate contrast-to-noise ratio forlesion detection. Hence, sampling at many diffusion weighting levelswith averaging would result in overly long scan times.

With the present innovation a largely overlooked benefit of fittingcomes into play, namely that for most encoding levels the noise-relatedvariance of a fit is much smaller than the noise-related variance of thedata the fit was applied to. Thus, for arbitrary weighting levels,signal values can be generated that exhibit less variation than theunderlying data. Consequently, time spent on signal averaging in aconventional protocol can instead be used for sampling of many diffusionweighting levels over an extended range. Sampling of a larger number ofsignals over a wide range of different diffusion weighting levels alsoopens the possibility to apply advanced fitting methods that allow noiseestimation and correction of Rician signal bias in areas of low signal.Any monoexponential sampling protocol can then be performedretrospectively by selecting data points along the fit, i.e., datapoints characterized by little noise-related variation, and thenperforming a monoexponential fit to these data points in order to obtainthe ADC. The diffusion-weighting level sampling pattern can be adjustedto control the variation of fitted signal values at particular levels,e.g., by changing sampling density or by signal averaging at a selecteddiffusion-weighting level. To some extent even the sampling range can bevaried, without impacting the resulting fit. This means that gradientnon-linearities at off-center locations can be accounted for, by simplyadjusting the nominal sampling points defined by the protocol. It alsomeans that sampling protocols can appropriately be varied to retainequal echo time and diffusion encoding time among MR systems withdifferent magnetic field gradient capability.

An apparatus and pulse sequence suitable for the practice of thisinvention, as will be seen from FIGS. 5 and 6 , includes an MRI system60, generally having a magnet assembly, interface circuitry, and acomputer 90. The magnet assembly includes a very strong magnet 63 thatcreates a homogeneous magnetic field within and around a sample (e.g. aninert sample or patient). X, Y, and Z magnetic field gradient coils 64,66, and 68 also form a part of the assembly and are positioned proximateor surrounding the sample 70. The assembly further comprises one or moreRF coils 72, which are positioned near or around the sample.

The interface circuitry 62 includes a gradient waveform generator 74with a control input connected to the computer and signal outputsconnected to the gradient coils 64, 66, and 68, as well as an RF signalgenerator 76 with a control input connected to the computer and anoutput connected to an input of an RF power amplifier 78. The RF poweramplifier has an output connected to an input of an RF switch 80. The RFswitch has an output connected to the RF coil 72 and an output connectedto the input of an RF detector 82.

The computer 90 includes computing hardware 92 and storage 94. Thecomputing hardware can comprise special purpose hard-wired computingcircuitry dedicated to MR acquisition and image reconstruction, as wellas a special programmed general purpose computer for display and userinteraction. The storage 94 can include various types of storage, suchas disk storage and random access memory. The storage can be used tostore data and programs, including programs used to interact with thesystem's interface circuitry 62. The computer has a video output forproviding video signals to a display 96, as well as control outputsconnected respectively to control inputs of the gradient waveformgenerator 74 and the RF signal generator 76. The computer also hasacquisition input operatively connected to an output of the RF detector82.

In operation, referring to FIGS. 5 and 6 , the imaging system 60 buildsimages under the control of the computer 90 according to a echo-planarimaging (EPI) sequence. At the beginning of an acquisition sequence foran image, the computer 90 sends a signal to the RF signal generator 76,which responds by generating a spatially and spectrally selective pulse8 with a flip angle of 90° or greater. This pulse is amplified by the RFpower amplifier 78 and provided to the RF coil 72 via the RF switch 80.As this pulse is being provided, the computer instructs the gradientwaveform generator 74 to drive the Z coil 68 with a multi-polar pulsegroup 6.

Next, the gradient waveform generator 74 provides a set of gradientpulse groups 10, 12, and 14 for the first part of diffusion encodingrespectively to the X, Y, and Z gradient coils 64, 66, and 68. After thefalling edge of the diffusion gradient signals and a wait time, which isinserted if needed to center the echo refocusing for the readoutprocess, the gradient waveform generator 74 provides a set of dephasinggradient pulses 16, 18, and 20 respectively to the X, Y, and Z gradientcoils 64, 66, and 68. Subsequently, a spatially selective 180°refocusing pulse 21 is provided to the RF coil 72, in much the same waythat the spatial-spectral selective pulse was. At the same time, thegradient waveform generator provides a rectangular pulse 22 on the Zgradient coil. Then, the gradient waveform generator 74 provides a setof rephasing gradient pulses 24, 26, and 28 of same amplitude andduration as the dephasing gradient pulses respectively to the X, Y, andZ gradient coils 64, 66, and 68. Then, the waveform generator provides aset of gradient signals 30, 32, and 34 for the second part of diffusionencoding respectively to the X, Y, and Z gradient coils 64, 66 and 68.Both parts of the diffusion encoding gradient signals to the X, Y, and Zgradient coils are scaled to attain a desired diffusion weighting anddiffusion encoding direction.

Once the gradient signals for diffusion encoding are turned off, thegradient waveform generator provides a set of dephasing gradient pulses44, and 46 respectively to the X and Y gradient coils 64 and 66. Thenthe gradient waveform generator provides gradient pulse group 48 on theX coil 64 for spatial encoding along the frequency encoding directionand at the same time gradient pulse group 50 on the Y coil 66 forspatial encoding along the phase encoding direction. As a result of thisexcitation sequence, a train of echoes 52 is received from the slice orvolume that was excited by the spatial-spectral selective RF pulse andthe 180° RF pulse. The RF coil receives these echoes and provides themvia the RF switch 80 to the RF detector, from where the digitizedsignals are forwarded to the data storage 94. After the signal echoeshave been received, optional crusher gradient signals 54, 56, and 58 canbe applied respectively to the X, Y, and Z gradient coils 64, 66 and 68.The computer 90 processes the signal data of the image echoes 52 and theimage for this diffusion encoding can be displayed on the display 96.

Scanning can be performed sequentially for a plurality of diffusionencoding gradient amplitudes and a plurality of diffusion encodingdirections. The sampling of diffusion-weighting levels can occur witheven b-factor increments, but other sampling schemes with non-uniformsampling density may also be considered. After obtaining images for alldiffusion encoding levels and all diffusion encoding directionsprocessing can be initiated to generate higher-order model fits to themeasured signal decay at each pixel position. This implies themeasurement of a sufficient number of diffusion encoding levels tosupport such higher order model fits. Then, for each pixel position, thesignal values estimated by the fit function for at least two predefineddiffusion weighting levels are used to fit a monoexponential diffusionsignal decay function with the ADC parameter as result. In pixels wherenoise or other errors prevent a meaningful calculation of ADC, aplaceholder value can be used instead. Maps of the ADC parameter andmaps of signals estimated by the higher-order model fit at diffusionweighting levels of particular interest can then be displayed on thedisplay 96 for diagnostic assessment.

For protocol planning it can be useful to understand the signalaveraging effect that arises by the higher-order model fit. AssumingGaussian noise, the averaging effect is a measure of how many repeatedacquisitions are necessary at a certain b-factor to attain the samereduction in noise standard deviation as observed with model fitting.The signal averaging effect can be estimated through repeated simulationusing a predefined signal decay function with added Gaussian noise σ_(g)followed by fitting with the higher-order model function. Ideally thesignal decay function is generated with the higher-order model functionusing predefined parameters, but this does not need to be case, as longas the evaluation is performed over a limited encoding range, wherenoise related deviations are significantly larger than deviationsbetween functions. The averaging effect AE as a function of the b-factorb can be defined as

${A{E(b)}} = \left( \frac{\sigma_{g}}{S{{TD}\left( {S_{fit}(b)} \right)}} \right)^{2}$with

${{STD}\left( {S_{fit}(b)} \right)} = \sqrt{{\frac{1}{N}{\sum\limits_{i = 1}^{N}{S\left( {b;{\theta}_{i}} \right)^{2}}}} - \left( {\frac{1}{N}{\sum\limits_{i = 1}^{N}{S\left( {b;{\theta}_{i}} \right)}}} \right)^{2}}$where STD(S_(fit)(b)) is the standard deviation for N model fits andθ_(i) the vector containing the model parameters that result withfitting of realization i.

Alternatively, the averaging effect can be calculated using ananalytical expression for the data error σ_(y)(b; a) proposed by Richter(The Telecommunications and Data Acquisition Progress Report, TDA PR42-122, April-June 1995, pp. 107-137), where a=[a₀, a₁, . . . , a_(M−1)]is a vector containing the true free model parameters (Min total) and bis the b-factor. The derivation of σ_(y)(b; a) in the case of non-linearleast square estimation rests on the assumption that the model functionand the least square error can be expressed as a Taylor expansion arounda of first and second order, respectively. This is important asconsiderations from the linear least square case are then applicable.The averaging effect AE in this case is then calculated as

${A{E(b)}} = \left( \frac{\sigma_{g}}{\sigma_{y}\left( {b;a} \right)} \right)^{2}$

With such analytic expressions it can be shown that for a sufficientlylarge number of samples the averaging effect grows approximatelyproportional to the number of samples. It also can be shown that themean of the averaging effect over all b-factors decreases proportionallywith the number of free parameters of the higher order model function.Thus, it is in general preferable to choose higher order-model functionswith a low number of free model parameters as long as the model functionis suitable for the data in question.

The prime application of the innovation is in diffusion image analysis,but it is equally applicable in the quantitative assessment of any MRrelaxation process that is characterized by non-monoexponentialrelaxation.

EXAMPLE

The signal averaging effect was simulated and analyzed analytically forthe biexponential, a kurtosis, and a gamma distribution model function.The signal data used for simulation relied on signals generated by twobiexponential tissue models. For simulation of prostate normalperipheral zone (PZ) tissue, the fast diffusion coefficient D_(f) wasset to 2.2 μm²/ms, the slow diffusion coefficient D_(s) to 0.4 μm²/msand the relative fraction f to 0.8. Meanwhile, for simulation ofprostate cancer tissue in the PZ, the fast diffusion coefficient D_(f)was also set to 2.2 μm²/ms, the slow diffusion coefficient D_(s) to alower value of 0.2 μm²/ms and the relative fraction f to a lower valueof 0.6. These model parameters were selected in approximate accordanceto literature values based on measurements that exclude the influence ofintra-voxel incoherent motion (IVIM). The signal data used forsimulation was generated by the two biexponential tissue models at N=21b-factors, evenly distributed in the range from 0 to b_(max). For theupper limit b_(max) values of 1500, 2000, and 2500 s/mm² were evaluated.Simulations were performed at SNR=20. For each simulation scenario,60000 signal data realizations were created by adding Gaussian noiseaccording to the simulated SNR. Each of the resulting signal decayprofiles was fitted with biexponential, kurtosis and gamma distributionmodel functions by means of non-linear least square estimation and theresulting signal values S(b) at the 21 assumed measurement b-factorswere calculated for each model function.

The analytic assessment of the averaging effect used the same tissuemodels. For the biexponential model function the biexponential tissuemodel parameters D_(f), D_(s), and f can directly be transferred to thea vector. For the kurtosis and gamma distribution model, approximatetrue model parameters a were determined by means of direct fitting ofthe biexponential tissue model data.

FIGS. 7A-C shows the averaging effect for b_(max) values of 1500, 2000,and 2500 s/mm², respectively. Vertical dashed lines indicate theb-factors that are relevant in clinical imaging, i.e., a high b-factorof 1000 s/mm² is typical for a brain exam and a high b-factor of 1500s/mm² is considered optimal to examine prostate lesions on adiffusion-weighted image. A good agreement of the averaging effectestimated with simulation (symbols) and analytic assessment (continuouslines) for simulated prostate normal (N) tissue and simulated prostatetumor (T) tissue is evident. An average of the approximate true modelparameters a determined for the different b_(max) values was used forthe kurtosis and gamma distribution model. Accordingly, with the highestb_(max) value, an obvious deviation between simulation and analyticassessment can be seen for part of the evaluated range of b-factors. Fora given model function, the overall shape of the AE curve appears to beindependent of the maximum b-factor. Thus, an increase in b_(max) leadsto a stretch of the curve over a larger b-factor range. For b=0,irrespective of higher-order model tested, the averaging effect is weak,i.e., AE≈1.

The same tissue models were also used to obtain simulated results ofapplying the innovation to obtain a retrospective monoexponential fit.The signal data used for simulation was generated by the twobiexponential tissue models at N=21 b-factors, evenly distributed in therange from 0 to 2000 s/mm². Simulations were performed at SNR=20. Foreach simulation scenario, 60,000 signal data realizations were createdby adding Gaussian noise according to the simulated SNR. Fitted signalvalues for each of the higher-order model functions at diffusionencoding levels 100, 1000, and 1500 s/mm² were used to applymonoexponential fits and to derive the ADC values. The choice of theseencoding levels was in accordance with an existing clinical protocol.Since the clinical protocol involved 6-fold averaging, 10,000realizations of clinical reference data were generated by 6-foldaveraging of the signal decay data realizations for the diffusionencoding levels 100, 1000, and 1500 s/mm². The following table shows anexcellent agreement between ADC values resulting with the conventionaland the proposed analysis that uses a retrospective fit. The nominal SNRis very similar, even though the retrospective fit was performed withunaveraged data. This reflects the influence of the averaging effect ofthe higher model fit. All results were obtained with non-linearregression analysis.

Normal Tissue Tumor Tissue Simulation ADC ADC Method Model [μm²/ms][μm²/ms] Clinical Monoexponential 1.39 ± 0.08 0.79 ± 0.04 (17.0) (18.0)Retrospective Kurtosis 1.40 ± 0.09 0.79 ± 0.06 Fit (15.3) (13.2)Retrospective Biexponential 1.39 ± 0.09 0.79 ± 0.06 Fit (15.5) (14.0)Retrospective Gamma 1.38 ± 0.09 0.79 ± 0.06 Fit Distribution (15.7)(13.6) All values given as mean ± standard deviation (nominal SNR =mean/standard deviation)

The innovation was also tested with diffusion-weighted images of theprostate. Subjects underwent an examination with the standard clinicalprostate MRI protocol on a 3 T scanner (Philips Achieva dStream, PhilipsHealthcare, The Netherlands) and a pelvic-phased array coil was used forsignal reception. Diffusion-weighted images were obtained according astandard clinical protocol at 3 mm isotropic resolution. Four b-factors,i.e., 0, 100, 1000, and 1500 s/mm² were acquired with 6-fold averagingusing single-shot echo-planar imaging. For each b-factor higher thanzero, images with diffusion encoding along each of three orthogonaldirections were collected. The total scan time for the diffusion scanwas 4 minutes and 4 seconds. The image acquisition for retrospective ADCanalysis using fitted data was performed with the same image geometryand diffusion encoding directions as the clinical protocol, but withoutsignal averaging. For each of three orthogonal encoding directions,diffusion-weighted images were collected for 21 evenly distributedb-factors ranging from 0 to 1500, 0 to 2000, or 0 to 2500 s/mm². Thetotal scan time was 4 minutes and 12 seconds, i.e., not significantlydifferent from the scan time of the clinical protocol. For the higherorder fit only values over the range 100 s/mm² to b_(max) wereconsidered. Maps of ADC were generated with the averaged clinical imagesat b-factors 100, 1000, and 1500 s/mm² and with the images computed withhigher-order model fits at the corresponding b-factors. An example of anADC map resulting with the clinical method is shown in FIG. 8A. Examplesof the corresponding ADC generated with this innovation usingnon-linear-regression are shown in in FIG. 8B, FIG. 8C, FIG. 8D,respectively. Regions of interest for the prostate peripheral zone andthe prostate transition zone were defined on the ADC map. The followingtable shows an excellent agreement between ADC values resulting with theconventional clinical approach and the innovation that uses aretrospective fit. Even if the higher-order model changes, there is aremarkable agreement. A bias correction was performed with OBSIDIAN(Optimized Bias and Signal Inference in Diffusion Image Analysis) and asexpected resulted in slightly higher ADC values.

Measurement Peripheral Tissue ADC Transition Zone ADC Method Model[μm²/ms] N = 25 [μm²/ms] N = 25 Clinical Monoexponential 1.54 ± 0.201.24 ± 0.11 Retrospective Fit Kurtosis 1.56 ± 0.21 1.25 ± 0.13Retrospective Fit Biexponential 1.53 ± 0.20 1.24 ± 0.11 RetrospectiveFit Gamma Distribution 1.53 ± 0.20 1.22 ± 0.11 Retrospective FitKurtosis bias corrected 1.62 ± 0.22 1.34 ± 0.11 N: Number of subjectsmeasured All values given as mean ± standard deviation

The measurement range can be varied without impacting the measured ADC,which is an important advantage of the innovation. FIG. 9A shows an ADCmap obtained with the clinical method. The ADC maps shown in FIG. 9B,FIG. 9C, and FIG. 9D were obtained with b_(max) values of 1500, 2000,and 2500 s/mm², respectively. Each of these ADC maps are practicallyindistinguishable from the clinical ADC map shown in FIG. 9A. The tablebelow shows that for regions of interest, the mean ADC values determinedon ADC maps that were obtained with different b_(max) values are notsignificantly different from the mean ADC value determined on theclinical ADC map.

Measurement Peripheral Zone ADC Transition Zone ADC Method Model[μm²/ms] N = 3 [μm²/ms] N = 3 Clinical Monoexponential 1.27 ± 0.06 1.16± 0.09 Retrospective Fit b_(max) = 1500 s/mm² Kurtosis 1.30 ± 0.06 1.19± 0.08 Retrospective Fit b_(max) = 2000 s/mm² Kurtosis 1.28 ± 0.06 1.17± 0.08 Retrospective Fit b_(max) = 2500 s/mm² Kurtosis 1.29 ± 0.06 1.18± 0.08 N: Number of subjects measured All values given as mean ±standard deviation

In conclusion, embodiments of the invention can provide a new and highlypractical technique to allow highly reproducible parameter mapping notonly of MR diffusion signal decays, but any MR relaxation process. Assuch, it should find broad applications in clinical diagnosis andresearch investigations, where parameter maps that are accurate, withminimal spatial and system dependence are highly desirable.

One skilled in the art will readily appreciate that the presentinvention is well adapted to carry out the objects and obtain the endsand advantages mentioned, as well as those inherent therein. The presentdisclosures described herein are presently representative of preferredembodiments, are exemplary, and are not intended as limitations on thescope of the invention. Changes therein and other uses will occur tothose skilled in the art which are encompassed within the spirit of theinvention as defined by the scope of the claims.

The invention claimed is:
 1. A method for producing parameter maps frommagnetic resonance images obtained at a multitude of contrast-weightinglevels, the steps comprising: a) selecting predefined contrast-weightinglevels; b) performing at each image pixel position a higher-orderfunction fit to the multitude of measured contrast-weighting leveldependent signals at said pixel position; c) determining for each saidpixel position computed signal values from said higher-order functionfit at said predefined contrast weighting levels; d) performing at eachsaid pixel position a monoexponential function fit through said computedsignal values related to said pixel position at said predefined contrastweighting levels and determine a monoexponential decay parameter; and e)assemble said monoexponential decay parameter of all image pixelpositions to a parameter map for display.
 2. The parameter mappingmethod of claim 1, wherein said measured contrast-weighting levelsextend over a wider range than said predefined contrast-weightinglevels.
 3. The parameter mapping method of claim 2, wherein the levelsof said measured contrast-weighting levels are arranged linearly.
 4. Themagnetic resonance imaging method of claim 3, wherein signal averagingis performed for selected levels of said multitude of measuredcontrast-weighting levels.
 5. The parameter mapping method of claim 2,wherein said higher-order function is applied to measurements ofdiffusion-weighted signal decays.
 6. The parameter mapping method ofclaim 5, wherein for each selected pixel position and for each measureddiffusion-weighing level, said higher order function fit is performedwith the spatially dependent, actual diffusion-weighting level insteadof a nominal diffusion-weighting level.
 7. The parameter mapping methodof claim 6, wherein system-specific adjustments of said measuredcontrast-weighting level range are made, to attain equal echo time anddiffusion encoding time for different magnetic resonance imagingsystems.
 8. The parameter mapping method of claim 7, wherein themagnitude signal of the complex diffusion-weighted signal is biascorrected.
 9. The parameter mapping method of claim 8, wherein thelevels of said measured contrast-weighting levels are arranged linearly.10. The magnetic resonance imaging method of claim 9, wherein signalaveraging is performed for selected levels of said multitude of measuredcontrast-weighting levels.
 11. The magnetic resonance imaging method ofclaim 10, wherein said higher model function fit uses a kurtosisfunction.
 12. The magnetic resonance imaging method of claim 10, whereinsaid higher model function fit uses a biexponential function.
 13. Themagnetic resonance imaging method of claim 10, wherein said higher modelfunction fit uses a function that is associated with a gammadistribution of the exponential decay parameters.
 14. The magneticresonance imaging method of claim 10, wherein said higher model functionfit uses a stretched exponential function.
 15. A magnetic resonanceimaging apparatus, comprising: a) means for providing images of signalsat a multitude of contrast-weighting levels; b) means for selectingpredefined contrast-weighting levels; c) means for performing at eachimage pixel position a higher-order function fit to the multitude ofmeasured contrast-weighting level dependent signals at said pixelposition; d) means for determining for each said signal pixel positioncomputed signal values from said higher-order function fit at saidpredefined contrast weighting levels; e) means for performing at eachsaid pixel position a monoexponential function fit through said computedsignal values related to said pixel position at said predefined contrastweighting levels and determine a monoexponential decay parameter; and f)means to assemble said monoexponential decay parameter of all imagepixel positions to a parameter map for display.